In the use of thermal energy to generate power and induce temperature changes, the costs of fuel sources and the generally recognized need for conservation of energy inevitably dictate that there be increased usage of thermodynamic machines. A number of different types of these machines are known, such as Stirling cycle machines used for power and refrigeration, and Vuilleumier cycle machines used for inducing hot or cold temperatures. A different thermodynamic cycle is disclosed by the present inventor in U.S. Pat. No. 3,698,182, issued Oct. 17, 1972 for "Method And Device For Hot Gas Engine Or Gas Refrigeration Machine". A more recent development is described and claimed in a presently copending application for patent of the present inventor, entitled "Unitary Heat Engine/Heat Pump System", filed Dec. 30, 1981, Ser. No. 335,659. Systems in accordance with this invention are capable of achieving substantial improvements in energy gain and coefficient of performance (COP) in deriving thermal outputs at intermediate temperature levels.
Thermodynamic machines can be constructed to operate with good thermal efficiency, and are capable of use for heating or cooling applications, or both. They are particularly attractive for energy conservation application, as described above in conjunction with the referenced patent application, because of their versatility and adaptability. They provide new opportunities for the potential use of solar energy, waste heat, and the heat content of ambient air, water and ground sources.
A distinction should be observed, however, between machines which induce thermal energy changes by using approximately constant volume thermodynamic cycles and those machines in which substantial pressure differentials exist and work is done by a power piston against an external medium. The essentially constant volume devices are exemplified by Vuilleumier devices, which are machines for inducing temperatures, and by the systems and methods disclosed in the previously referenced patent application. The latter may be characterized as heat pumps for energy gain. In contrast, the Stirling cycle machines create significant pressure differentials across a power piston and significant change in the internal volume (per work cell in a multicylinder system) and may be distinguished as heat engines.
Constant volume devices are particularly interesting for new applications, because of their versatility and reliability. Because they do not create substantial pressure differentials, they do not present the sealing problems and mechanical load problems that arise with Stirling cycle machines, and they can be very large, as well as reliable and maintenance free over long periods. Because they employ approximately constant volume displacement, they are particularly dependent upon the temperature ratio, the internal void (dead) volume, and the effectiveness of the regenerator in the system. The temperature ratio between the hot and cold ends of the regenerator, based upon absolute temperatures, is primarily determinative of system performance, particularly in terms of specific output. At low temperature ratios (e.g. substantially less than 2 to 1) the heat density for a given design and dead volume is usually unacceptably low. However, significant amounts of thermal energy may be available from intermediate level sources at below about 300.degree. C. If one is to realize the benefits of these constant volume systems in using the heat content from solar, ambient and waste heat sources, therefore, it becomes extremely important to confront the problem of the temperature ratio limitation. The Knoos system described in the referenced patent application, for example, shows a number of noteworthy applications of a thermodynamic system in which the coefficient of performance can be substantially improved over prior art systems with comparable inputs. The description also demonstrates, however, that the system is dependent, in terms of specific energy (heat) output, on pressure and temperature ratios. Heretofore it has not been feasible to use thermodynamic machines of the constant volume type where temperature ratios are low. The temperatures of the heat sources are established by conditions of availability and are effectively immutable; the pressure ratio that then results may be so low that the machine operates with very low efficiency and specific energy (heat) output.